Depending on the specific PHP topic that you’re wanting to learn, I might be able to recommend a specific course, because I’ve used a ton of different platforms to learn PHP in recent years and up until now.
+1 on Laracasts, which is extensive and has a lot of topical depth on different aspects of PHP. But depending on what you’re wanting to learn, Lynda and Pluralsight both have their advantages. Lynda doesn’t have a ton of PHP content, but there are some courses on there that are more useful than others. Likewise with Pluralsight.
If you don’t mind ebooks (which I’d recommend over online courses, simply because you can read through an ebook faster than a video course), SitePoint Premium has some very good ebooks that you can nab on their 7-day trial. Their Jump Start PHP is a good one that covers a lot in not too many pages, or if you’re looking for something more gradual & in-depth, their PHP & MySQL: Novice to Ninja is better. But you might as well download all of their PHP ebooks if you sign up for their trial.
Sometimes there’s also no substitute for a good printed book, and the best I’ve seen are those by Larry Ullman. Either PHP for the Web: Visual QuickStart Guide if you’re looking for PHP only, or PHP and MySQL for Dynamic Web Sites: Visual QuickPro Guide if you don’t mind learning MySQL at the same time.